There is a method of diffracting light having various wavelengths into different directions to separate the light depending on the respective wavelengths, and measuring the intensity of each of the separated light to determine the intensity of light of various wavelengths contained in the incident light. Here, in order to utilize the separated light with good efficiency, it is preferred that a diffracted light is efficiently concentrated on a specific diffraction order, and as the method for the wavelength separation, a method of employing a diffraction element having a saw-tooth-like cross section is known.
In order to increase the angle between the incident light and the diffracted light to increase the flexibility in disposing optical elements, it is preferred to use a transmission type diffraction grating. FIG. 7 shows a conventional transmission type diffraction grating employing a resin. The diffraction element is formed by the following method. Namely, by pressing a mold, not shown, having a straight saw-tooth-form diffraction grating precisely formed therein, against a photo-sensitive resin 702 deposited on a surface of a glass substrate as a transparent substrate 701, curing the photo-sensitive resin 702 with UV light, and separating the mold from the resin to form a saw-tooth-form diffraction grating 703, whereby a transmission type saw-tooth-like diffraction element 70 is obtained.
Further, FIG. 8 shows an example of a transmission type pseudo-saw-tooth-like diffraction grating whose saw-tooth-form is approximated by steps. The pseudo-saw-tooth-like diffraction grating 802 is formed by repeating a photolithography method and an etching method to a surface of a glass substrate as a transparent substrate 801, and the diffraction grating is used as a diffraction element 80 of transmission type having a pseudo-saw-tooth form.
Here, an arrow mark of solid line shows incident light, an arrow mark of dashed line shows transmission light, and an arrow mark of broken line shows the −1st order diffraction light.
In the above conventional example, in order to obtain a high diffraction efficiency with a transmission type saw-tooth-like diffraction grating, it is necessary that the phase difference defined as a product of the refractive index difference between the grating material and output-side medium such as an air, multiplied by the depth of the grating, is determined to be about the wavelength of light. Meanwhile, the period of the grating is preferably small to increase the separation angle depending on wavelength. For these reasons, the shape of the saw-tooth-form diffraction grating has to be such that the period of grating is small and the aspect ratio (D/T) defined as the ratio of the grating depth D to the grating period T, is large.